Several trends presently exist in the semiconductor device fabrication industry and in the electronics industry. Devices are continually getting smaller, faster and requiring less power, while simultaneously being able to support a greater number of increasingly sophisticated applications. One reason for these trends is that there is an ever increasing demand for small, portable and multifunctional electronic devices. For example, cellular phones, personal computing devices, and personal sound systems are devices which are in great demand in the consumer market. These devices rely on one or more small batteries, which are generally rechargeable, as a power source and also require an ever increasing storage capacity to store data, such as digital audio, digital video, contact information, database data and the like.
To achieve these and other ends, a continuing trend in the semiconductor manufacturing industry is toward producing smaller and faster transistor devices, which consume less power and provide more memory density. Integrated circuits (ICs) are thus continually designed with a greater number of layers and with reduced feature sizes and distances between features (e.g., at sub micron levels). This can include the width and spacing of interconnecting lines, the spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. The scaling-down of integrated circuit dimensions can facilitate faster circuit performance, more memory and can lead to higher effective yield in IC fabrication by providing more circuits on a die and/or more die per semiconductor wafer.
Semiconductor based products (e.g., DSP's, microprocessors) can include and be utilized on a variety of different items including one or more different types of memory, such as static random access memory (SRAM), dynamic random access memory (DRAM) and/or embedded memory, as well as logic such as latches, flip-flops and/or combinatorial logic that interconnects memory to cache(s). Respective types of memory generally include thousands or millions of memory cells, adapted to individually store and provide access to data. A typical memory cell stores a single binary piece of information referred to as a bit. The cells are commonly organized into multiple cell units such as bytes which generally comprise eight cells, and words that may include sixteen or more such cells, usually configured in multiples of eight. Storage of data in such memory device architectures is performed by writing to a particular set of memory cells, sometimes referred to as programming the cells. Retrieval of data from the cells is accomplished in a read operation. In addition to programming and read operations, groups of cells in a memory device may be erased.
The erase, program, and read operations are commonly performed by application of appropriate voltages to certain terminals or nodes of the cells. In an erase or program operation the voltages are applied so as to cause a charge to be stored in the memory cells. In a read operation, appropriate voltages are applied so as to cause a current to flow in the cells, wherein the amount of such current is indicative of the value of the data stored in the respective cells. The memory devices include appropriate circuitry to sense the resulting cell currents in order to determine the data stored therein, which may then be provided to data bus terminals for access by other devices in a system in which the memory device is employed.
As the dimensions and operating voltages of electronic devices are reduced to satisfy the ever-increasing demand for higher density and lower power, their sensitivity to radiation increases dramatically. Radiation can, directly or indirectly, induce localized ionization events capable of upsetting internal data states. While the upset causes a data error, the circuit itself is undamaged; thus this type of event is called a “soft” error and the rate at which these events occur is called the soft error rate (SER). It has been established that SER in semiconductor devices is induced by three different types of radiation; alpha particles, high-energy neutrons from cosmic radiation, and/or the interaction of cosmic ray thermal neutrons and 10B in devices containing borophosphosilicate glass.